Among the primary goals of total hip replacement are the elimination of pain and the restoration of function. Cemented total hip replacement has been a well accepted and successful method for the treatment of the severely degenerated arthritic hip. Recent developments in the use of biological fixation have led to design and material changes in total hip replacements, with the goal of increasing implant longevity and, thus, making the procedure available to the younger patient. At present, the functional and biomechanical implications of these design changes are not well understood. Early reports of patients with cementless devices suggest an apparent higher incidence of prolonged pain, reduced function and an unexplained limp without associated pain in some patients with cementless fixation. The meaning, cause and implication of these functional differenced are not known currently, although they are issues of concern. The purpose of this investigation is to analyze several specific hypotheses related to factors influencing function following total hip replacement. The first and third Specific Aims address the hypothesis that either micromotion or transfer of load can be sensed and, thus, cause functional adaptations. The second Specific Aim examines the hypothesis that the position of the femoral head relative to the pelvis and to the femur have a substantial influence on patient function. The methodology and instrumentation will include a gait analysis system designed for biomechanical evaluation of function previously used by this laboratory. Kinematics and kinetics of the hip, knee, and ankle will be evaluated bilaterally during walking, stairclimbing and rising from a seated position. In addition, CT scans will be used to determine the femoral need geometry and evaluate the fit of the femoral stem in the medullary canal. Patients will be evaluated pre-operatively, at one year, two years and three years post-operatively. Femoral heads retrieved at the time of surgery, will be used to provide cartilage material to the projects in the SCOR. The information generated by this study will provide a better estimate of the hip joint loads that occur during walking and other functional activities in patients with cementless devices. It is possible that these loads may be substantially different and have a significant influence on the stresses sustained by both the bone and implant. Another possible significant outcome of this study is that the functional adaptation used by some patients maybe a beneficial predictor of long-term potential outcome.